El Niño weather conditions cause torrential downpours and crop-killing droughts in the United States, but a study released last month theorizes that this hot-climate cycle can contribute to far deadlier outcomes in parts of the developing world.

The study, published Aug. 24 in the journal Nature, said the probability that civil wars will break out in 90 tropical countries such as Burma and Colombia doubles in years when the El Niño Southern Oscillation warms the climate, compared with cooler La Nina years.

The study “raises potent questions” about the impact of weather cycles as the climate warms, the authors said.

“We can speculate that a long-ago Egyptian dynasty was overthrown during a drought. That”s a specific time and place … so people might say, ”OK, we”re immune to that now,”” said Solomon Hsiang, a graduate of Columbia University”s Earth Institute doctorate program in sustainable development and the study”s lead author. “This study shows a systematic pattern of global climate affecting conflict and shows it right now.”

El Niño Southern Oscillations are huge climate events that happen every four to seven years. They occur when powerful trade winds in the Pacific push colder water down and move warmer surface water atop it. The cool winds weaken, making the water even warmer.

The climate heats up, producing heavy rain in some areas and droughts in others. The conditions ruin agriculture, the main source of income in poor tropical nations. The most recent El Niño was in 2009, and one of the most powerful occurred in 1997 and 1998. (A strong El Niño can last more than a year.)

Authors of the study stopped short of claiming that El Niño can ignite wars, asserting instead that it can be a major contributing factor, along with economic strife and competing political and religious ideologies that split nations.

Halvard Buhaug, a senior researcher at Oslo”s Centre for the Study of Civil War, dismissed the study, saying it fails to clearly show how El Niño drives governments and their opponents to civil war.

“It would boost the credibility of the claim if a proper theory were developed, pointing to specific causal mechanisms linking warming of the … Pacific with civil war outbreak,” Buhaug said. “These are obvious challenges for future research. For now, I find it advisable to exercise restraint in embracing the notion of a causal El Niño-civil war connection.”

Hsiang said Buhaug is right: There are no specific examples in the study.

“We only wanted to speak to what we can prove,” Hsiang said. He compared his team”s findings to the first studies that tied cigarettes to cancer without specific linkages, prompting more studies.

Hsiang and two fellow researchers tracked the El Niño Southern Oscillation for more than half a century, from 1950 to 2004. They studied 175 nations and 234 civil wars, half of which resulted in more than 1,000 deaths. Ninety of those nations were in the tropical zone affected by El Niño.

They concluded that El Niño appeared to play a role in 30 percent of the civil conflicts that occurred in tropical nations, where weather is more strongly influenced by El Niño”s climate changes. During La Niña, the chance of civil war breaking out was about 3 percent; during El Niño, the chance doubled, to 6 percent.

“El Niño produces hotter and drier conditions . . . increased risk of natural disaster and hurricane activity, and it”s costly,” said Kyle Meng, a co-author of the study. It affects labor markets, increases unemployment and hampers the ability of governments to enforce law. “As a result, we observe an increase in violence,” said Meng.

Meng said the study documents that “when El Niño occurs, there”s large decreases in crop yields as well as in total income attributed to agriculture” in countries that rely heavily on farming food and cattle.

The researchers said they focused solely on fighting between central governments and opposition groups rather than hostilities between nations because civil conflicts represent the vast majority of wars around the world.

Tropical nations that were caught up in wars when El Niño was present include Sudan, El Salvador, the Philippines, Uganda, Angola, Haiti, Burma, Eritrea, Indonesia, Cambodia and Rwanda.

“I would love to say we would be able to predict conflicts, but this study falls short of that,” Meng said. “We are able to predict strong El Niño years, and based on the result of our study the likelihood of violence breaking out in the tropics (during those years) increases dramatically. At a minimum, national governments and national institutions should be ready for such a thing.”